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The reactivity of the isomeric oxo-Mn(V)-2-tetra-N-methylpyridyl (2-TMPyP) and oxo-Mn(V)-4-tetra-N-methylpyridyl (4-TMPyP) porphyrins has been investigated by a combined experimental and theoretical approach based on density functional theory. The unusual higher reactivity of the more electron-rich 4-TMPyP species appears to be related to both the higher basicity of its oxo ligand, compared to that of the 2-TMPyP isomer, and the smaller low-spin-high-spin promotion energy of 4-TMPyP, compared to that of 2-TMPyP, because of the stabilization of the A2u orbital in the latter isomer. Therefore, in a two-state energy profile involving crossing of the initial singlet and final quintet potential energy surfaces, the 4-TMPyP isomer should be kinetically favored. The calculated differences in the singlet-quintet gaps for the 2-TMPyP and 4-TMPyP systems compare well with the measured differences in the activation energies for two isomeric porphyrins. Both effects, proton affinity and electron-promotion energy, contribute to reduce the reactivity of the more electrophilic oxidant when electron-withdrawing groups are closer to the active site, contrary to the usual expectations based on simple chemical reactivity correlations. These theoretical results are in accord with new experimental data showing O=Mn(V)-O-H pK(a)s of 7.5 and 8.6 for the isomeric 2-TMPyP and 4-TMPyP systems, respectively.

Transition metal oxo-species have been the focus of extensive studies because of their relevance to the redox biochemistry of dioxygen as well as myriads of oxidative catalytic processes. High valent oxo-manganese complexes have been described for porphyrin, 1 salen, 2 corrole, 3 corrolazine, 4 and non-heme systems. 5 The O=Mn V moiety has been suggested in the photosynthetic water oxidation process, 6 and a bridged Mn V porphyrin dimer has recently been demonstrated to oxidize water into dioxygen. 7 We have previously described low-spin d 2 oxomanganese(V) porphyrin complexes that display an extraordinary range of reactivity toward oxo-transfer as a function of prototropic equilibria involving the axial ligand. 8 A prediction of that work was that oxo-aqua and oxo-hydroxomanganese(V) intermediates are reactive oxidants while the stable species observed at high pH are trans-dioxo complexes. Here we provide the first definitive spectroscopic evidence for trans-dioxomanganese(V) porphyrins [O=Mn V =O]. Further, we show that protonation of these species affords the reactive intermediates usually associated with these catalytic systems (Scheme 1).

Synthetic manganese porphyrins and related systems have been used extensively in chemical modeling of biological monooxygenation reactions catalyzed by heme proteins. [1] They are also versatile catalysts for the oxygenation of alkanes, alkenes, and nitrogen-and sulfur-containing compounds using oxygen donors such as iodosylbenzene, sodium hypochlorite, alkyl, aryl, and hydrogen peroxide, amine Noxides, and molecular oxygen. [2] Only recently has the key oxomanganese(v) intermediate been well characterized. [3] Here we report that the oxomanganese(v)-5,10,15,20-tetrakis(N-methyl-2-pyridyl)porphyrin (1) can efficiently transfer its oxo ligand to bromide ion, and that this oxo transfer is rapid and reversible. [Eq.(1)]The forward reaction mimics the halide oxidation reaction catalyzed by haloperoxidases, [4] while the reverse reaction is the catalyst activation step in substrate oxygenation by manganese porphyrins. This well-behaved equilibrium allows the assignment of a free energy change for the reaction depicted in Equation (1).OxoMn V TM-2-PyP (1) has unusual stability in aqueous solution compared to other oxoMn V porphyrin intermediates. [3b] It can be generated by the stoichiometric reaction of Mn III TM-2-PyP [5] (2) with oxidants such as HSO 5 À , m-CPBA (chloroperoxybenzoic acid), and OCl À . We have found that hypobromite, a weaker oxidant, [6] is also able to generate 1 smoothly. Figure 1 a shows the reaction between 5 mm 2 and 50 mm HOBr/OBr À[7] at pH 8.5 monitored by stopped-flow spectrophotometry. [3a,b] Clear isosbestic points were observed at 392, 444, and 558 nm. Remarkably, the identical isosbestic behavior was also found for the reverse reaction, oxoMn V Br À at higher bromide concentration. Typical spectral changes observed for the oxo-transfer reaction from Figure 1. Time-resolved UV/Vis sepctra for the reaction of a) 5 mm Mn III TM-2-PyP (2) and 50 mm HOBr/OBr À ; b) 5mm Mn V TM-2-PyP (1) and 50 mm Br À at pH 8.5 (10 mm Na 2 B 4 O 7 /H 2 SO 4 buffer). For both reactions there were 60 scans in 120 ms. Every fourth scan is shown.1 to Br À are shown in Figure 1 b. The generation of hypobromite, which is favored by excess bromide and lower pH, was confirmed by observing the diagnostic bromination reaction of phenol red. [8] The pH dependence of the rate of oxo transfer to bromide was examined between pH 5.2 and 9.0 (I 0.25 m NaClO 4 ). The reaction was found to be first-order in both oxoMn V and Br À , and independent of the buffer concentration. Kinetic profiles were obtained by monitoring oxoMn V (1) at 433 nm. Pseudo-first-order fitting of the kinetic data to a single exponential was carried out with at least six concentrations of Br À at each pH value. The apparent second-order rate constant k app was calculated from the slope of the linear plot of k obs versus C Br À . Our results show that 1 was nearly as effective an oxygen donor to Br À (3.8 Â 10 5 m À1 s À1 at pH 7.0) as myeloperoxidase compound I (1.1 Â 10 6 m À1 s À1 at pH 7.0), [9] and much more effective than vanadium bromopero...

A water-soluble manganese porphyrin, 5,10,15,20-tetrakis-(1,3-dimethylimidazolium-2-yl)porphyrinatomanganese(III) (Mn(III)TDMImP) is shown to react with H(2)O(2) to generate a relatively stable dioxomanganese(V) porphyrin complex (a compound I analog). Stopped-flow kinetic studies revealed Michaelis Menton-type saturation kinetics for H(2)O(2). The visible spectrum of a compound 0 type intermediate, assigned as Mn(III)(OH)(OOH)TDMImP, can be directly observed under saturating H(2)O(2) conditions (Soret band at 428 nm and Q bands at 545 and 578 nm). The rate-determining O-O heterolysis step was found to have a very small activation enthalpy (ΔH(≠) = 4.2 ± 0.2 kcal mol(-1)) and a large, negative activation entropy (ΔS(≠) = -36 ± 1 cal mol(-1) K(-1)). The O-O bond cleavage reaction was pH independent at 8.8 < pH < 10.4 with a first-order rate constant of 66 ± 12 s(-1). These observations indicate that the O-O bond in Mn(III)(OH)(OOH)TDMImP is cleaved via a concerted "push-pull" mechanism. In the transition state, the axial (proximal) (-)OH is partially deprotonated ("push"), while the terminal oxygen in (-)OOH is partially protonated ("pull") as a water molecule is released to the medium. This mechanism is reminiscent of O-O bond cleavage in heme enzymes, such as peroxidases and cytochrome P450, and similar to the fast, reversible O-Br bond breaking and forming reaction mediated by similar manganese porphyrins. The small enthalpy of activation suggests that this O-O bond cleavage could also be made reversible.